In helical CT scanning, the term 'pitch' is defined as the ratio of table feed per rotation to total beam collimation. If pitch is increased from 0.8 to 1.6 while other parameters remain constant, what happens to the radiation dose to the patient?

• A The dose doubles
• B The dose remains unchanged
• C The dose is approximately halved ✓
• D The dose increases by a factor of four

Explanation

For a given mAs per rotation, increasing pitch spreads the same tube output over a longer scanned length, reducing dose approximately inversely with pitch. Doubling pitch roughly halves the patient dose (dose ∝ 1/pitch when mAs/rotation is fixed). Many modern CT scanners use effective mAs (mAs/pitch) which keeps dose constant across pitch changes, but in the classic definition with fixed mAs/rotation, dose falls as pitch rises.

Reference: Bushberg, The Essential Physics of Medical Imaging, 3rd ed.

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